Compounds may be collected as they elute from liquid chromatographic columns ("LC columns") and then subjected to further analysis as, for example, by mass spectrometry ("MS"). For complex mixtures, the chromatographic column may be coupled directly with a mass spectrometer or other analytical apparatus. Where LC and MS are carried out as a unified, continuous process, it is known as liquid chromatography/mass spectrometry ("LC/MS").
Typically the effluent from the LC column is in a highly dilute liquid phase, with the separated components emerging from the LC column as "peaks" entrained within a liquid carrier. This can present significant problems for efficiently coupling the effluent with the ion source of the mass spectrometer. Nonvolatile inorganic components present in the eluent, for example as buffers, can interfere with ionization, and where gradient elution is employed the composition of the solvent changes in the course of the analysis. Typical flow rates of LC eluents produce, after vaporization, the equivalent of gas flows much too high for conventional ion sources to accommodate. The difficulty of removing the liquid solvent from the components to be analyzed in the mass spectrometer has raised significant challenges for the development of LC/MS systems.
Generally, in a conventional LC/MS analysis, the sample to be analyzed is separated using an LC column, and the eluent is directed to the mass spectrometer, which completely consumes the sample stream. Optionally, the LC eluent may be passed through a nondestructive detector, such as an optical detector, to provide general information as to the presence of peaks in the flow, without destroying the sample in the stream, before the sample stream is sent on to the MS. Optical detectors conventionally include, for example, UV-Vis, Refractive Index, and fluorescence detectors.
It may be desirable to employ a fraction collection system, to physically collect a purified sample or samples from a mixture in the LC eluent stream. In one conventional approach, a nondestructive detector is employed upstream from the fraction collector, and the time delay between the moment a peak is detected at the detector and the time of arrival of the peak at the fraction collector is calculated from the known or estimated flow rate and the measured or calculated volume capacity of the tubing and connections between the detector and the fraction collector. This calculated time delay provides a basis for an expectation of when a component of the sample, detected at the nondestructive detector, should arrive at the collection vessel. The sample component is presumed to be contained in a collection vessel that received the eluent flow at the estimated arrival time, and the material in that collection vessel can be selected for further analysis.